MacBook Pro :: It Crashes Intermittently - How To Stop It

Here's my hardware, purchased in September of 2009. Model Name: MacBook Pro  Model Identifier:MacBookPro5,3  Processor Name: Intel Core 2 Duo  Processor Speed:2.8 GHz Number Of Processors: 1 Total Number Of Cores: 2  L2 Cache: 6 MB  Memory:4 GB Bus Speed:1.07 GHz  Boot ROM Version:MBP53.00AC.B03 SMC Version (system): 1.48f2 

I'm running OSX 10.6.8. and have only used approximately 200GB of my 500 GB Hard drive. My mbpro simply hangs. I'll open a browser, or hot corner my spaces, save a file, whatever, and my laptop will not respond. Sometimes my cursor will still be movable but won't actually activate anything. I frequently get the BBOD and all I can do is wait until it stops and then all the cursor commands and keyboard clicks I made while it was spinning, execute...or not.I have run the Apple Hardware Test which came up negative. I've downloaded and installed ClamXAV which only came up with a couple of domain spoof type viruses which I've quarantined and deleted. I've got Little Snitch telling me what connections are going out but it seems that there's nothing unusual, but I frankly don't know that much about the processes other than what I have looked up on goole.I have looked at the Activity Monitor and every not and then, right when my machine hangs, I'll see a spike in Network Activity pane but almost seems as if the spike isn't being read because the processes that come up seem to be legitimate and often, they don't show that they've taken up all the bandwidth. Please help. I've got a G4 quicksilver from 2002 that still kicks A and my MBPro has made me nervous ever since switching to the Intel chipset. (Don't know if that actually has anytghing to do with it.) Lastly, I think there's a chance that I'm the fool who opened or executed something downloaded from the internet. It could have been a dashboard widget or an extension or add on or software or whatever.

Info:
Mac OS X (10.6.8)